||OBSTACLES TO REDUCE THE WIND AND DEVIATE THE SAND
Definition characters description and diffusion
The establishment of live fences, shelter belts and windbreaks provide several benefits, such as protection against wind erosion, and sand drifting. They reduce the wind speed and evaporation rates and hence ameliorate the microclimate and increase crop yields. The size, porosity, morphology and composition of fences vary between places but operate on the same principle of creating a barrier and producing areas of reduced sand-carrying capacity ahead of and behind the vegetation belts.
General characters description and diffusion
Establishment is contingent on the availability of drought-tolerant woody plants and application of suitable planting techniques. Sand binder species with tolerance to hot temperatures, aridity and infertile soils, fast growth, large crowns and well-developed root systems have provided good results. Multipurpose fence belts incorporating a combination of perennial herbs mixed with shrubs and trees under arrangements and densities that allow each component to contribute its desirable traits fully are considered advantageous. Woody multipurpose trees and shrubs, if used, provide a supplementary source of forage and fuel wood from pruning and soil improvement. Live fences of proven species may also be established alongside a stabilized mechanical structure. Wood lots of multipurpose trees and shrubs established between the edge of the sand and a given property also fix dunes while providing forage and other utility products. The rehabilitation of saline environments that are common in drylands is also dependent on successful establishment of a vegetative cover. Depending on the site conditions, grasses or trees may be suitable. In severe cases, "pioneer" species are often used to ameliorate the site sufficiently before the more desirable plants can be established. There are many isolated cases of successful application of biological measures. In Bouza in southern Niger, every street is lined with trees, and the town is being encircled by woods. Green belts are being planted around the capital cities of Ouagadougou and Niamey in Burkina Faso and Niger. The Tunisia-Morocco green belt is also noteworthy. Integrated programmes for sand dune fixation on the northern littoral and the protection of adjacent market gardens in Senegal, and regional programmes for sand dune fixation in Mauritania, the State of Rajasthan in India, North Africa, the Middle East, and the former states of the USSR are important landmarks demonstrating the efficacy of road-side shelter belts in protecting road and rail networks. Windbreaks are also of considerable importance where they have been established, on the large plains and windy southern part of South America, such as Patagonia. Sudan is restocking its gum belt, which acts as a barrier to the desert, while Peru has an ambitious programme for the rehabilitation of its Andean sierra. Irrigated agriculture in these countries would be inconceivable without the protection of windbreaks to afford protection from the hot winds and sand dunes. An irrigation project in Tunisia with wooded surrounds has increased agricultural yields many times over. The application of windbreaks for the protection of agricultural crops is highly developed in the drylands of China in order to protect crops from desiccation and physical damage from wind-blown sand and loess. Vast areas of farmland and villages in northern China which had been buried by shifting sands were rehabilitated from the mid 1950s onward through these technologies. In this instance straw barriers were erected at the foot of mega dunes and inter-dune depressions, and vegetation belts subsequently established behind the straw checkerboard. Consequently the southward movement of sand dunes was completely controlled and large areas of farmland were brought under effective protection by the shelterbelts.
Advantages and sustainability
Protection against wind and erosion and against sand drifting is achieved through the establishment of live fences, shelter belts and windbreaks. Wind speed and evaporation rates are reduced and crop yields are increased.